Nanotechnology is a fast developing field that has attracted attention. One of the important applications of nanotechnology is the manufacturing of nanocomposites. In fact, “the real interest in nanotechnology is to create revolutionary properties and functions by tailoring materials and designing devices on the nanometer scale”. (The Department of Chemistry, Michigan State University, “Nanocomposites—Classification Types, Potential Applications, Interactions and Novel Nanocomposites”. Azom. 1 Nov. 2004<http://www.azonano.com/details.asp/ArticleID=1283>) The term “nanocomposites” implies that one of the phases of the composite material (the matrix or the reinforcement phases) is composed of particles or fibers with nano-dimensions. The main advantage of nanocomposites is their vastly improved mechanical properties with a relatively small content of the filler material. This is mainly due to the large surface area of the filler material. Moreover, nanocomposites show other enhancements depending on the filler and the matrix elements. For instance, carbon nanotubes (CNT's) are used in polymer and ceramic matrices to produce electrically conductive nanocomposites. CNT's are also used in combination with metals to make use of the outstanding mechanical properties of the CNT's and the ductility of metals like aluminum and copper (Mora et al., 2009).
Clay nanocomposites represent a class of nanocomposites in which the filler element is nanoclay. These materials are known as smectite clays, such as hectorite, montmorillonite (MMT), and synthetic mica. Smectite clays are peculiar in their structure as they are composed of layers. Each layer is built from tetrahedrally coordinated Si atoms fused into an edge shared octahedral plane of Al(OH)3 or Mg(OH)2 (Sinha Ray and Okamoto, 2003). The mechanical properties of these individual layers are not yet known. However, some attempts have been made to model the mechanical properties of the silicate layers estimating the Young's modulus along the layer direction to be 50-400 times higher than that of a typical polymer (Gao, 2004).
Mechanisms of Clay Dispersion
There are two mechanisms associated with the dispersion of the silicate layers in the polymer matrix, intercalation and exfoliation (Sinha Ray and Okamoto, 2003). The intercalation mechanism involves the insertion of polymer molecules between silicate layers. The interlamellar spacing of the clay particles is an important parameter in this case. After intercalation, the effective size of the clay particles is larger as the volume of the clay particle increases. In exfoliation, silicate layers are dispersed in the polymer matrix. The thickness of each layer is 1 nm, while its length is in the range of microns. FIG. 1 shows the structure of the exfoliated and intercalated nanocomposites versus the conventional microcomposite
Advantages of Clay Nanocomposites
The main advantage of clay/polymer nanocomposites over fiber-reinforced polymeric composites lies in the possibility of producing composites with enhanced mechanical properties (stiffness, yield strength, wear resistance) with very low clay content. Much experimental work has been done with clay content ranging between 3-6 wt % (Sinha Ray and Okamoto, 2003; and, Hasegawa et al., 2003).
Clay nanocomposites with exfoliated silicate layers show improved resistance to organic solvents (alcohols, toluene, and chloroform) (Fengge, 2004).
Clay nanocomposites manufactured by Toyota showed an increase in the heat distortion temperature (87° C.) and a 45% reduction in the thermal expansion coefficient. This was believed to be due to the reduction in molecular mobility in the polymeric matrix (Fenegge, 2004).
Another advantage of clay nanocomposites is their flame retardant properties. The peak heat release rate during combustion of nylon6 was found to be reduced by 63% for 5 wt % of clay (Fenegge, 2004).
Moreover, clay nanocomposites, in contrast to conventional composites, have good optical properties. In conventional polymeric composites, the composite tends to be opaque due to light scattering at the reinforcing phase whether fibers or particulates. However, silicate layers do not affect the optical properties of the polymer matrix and, therefore, the resulting composite is transparent. This has been explained using two different interpretations. First, the thickness of the exfoliated clay layers is much less than the wavelength of light thus allowing the light to pass without scattering. Second, in nanocomposites, the size of the reinforcement is in the nanoscale, which allows the formation of composites at the molecular level (Fenegge, 2004).
Commercial Applications
Clay/polymer nanocomposites have been used extensively for industrial applications most of which are automotive applications. In 1991, Toyota and Ube manufactured timing belt covers out of clay/nylon6 nanocomposite. FIG. 2 shows a timing belt cover made from nylon/clay nanocomposite. This was followed by Unikita's attempt to manufacture engine covers for Mitsubishi's engines out of clay/nylon6 nanocomposites. In 2001, clay/polyolefin nanocomposites were used by General Motors and Basell as a step assistant for GMC Safari and Chevrolet Astro vehicles. Shortly after this, nanocomposites were used for producing the doors of Chevrolet Impalas. Recently, clay/polypropylene nanocomposites were used by Noble Polymers in the manufacturing of the seat backs of Honda Acura (Fenegge, 2004; and, Okada and Usuki, 2006).
Moreover, other applications have been developed trying to make use of the inherent property of clay/polymer nanocomposites as gas barriers. Alcoa Closure Systems International has developed multilayer clay/polymer nanocomposites as a barrier for enclosure applications. Moreover, Honeywell produced clay/nylon6 nanocomposites for commercial products. Recently, nylon-MXD6 nanocomposites have been developed by Mitsubishi Gas Chemical in collaboration with Nanocor for PET products (Fenegge, 2004).
Limitations
Generally, there are two main challenges associated with the fabrication of nanocomposites. The first one is to maintain a good dispersion of the nanoparticles inside the matrix. As the particles get smaller in size, they tend to agglomerate. This agglomeration creates stress concentrations and yields non-uniform distribution of the load on the nanocomposites, which does not give the expected properties in the resulting nanocomposites. The second challenge is maintaining a good bonding at the interface between the matrix and the reinforcement phases. The lack of such bond would not allow load transfer from the matrix to the reinforcement. Organically modified clays have been used to compatibilize the polymer matrix with the clay in order to overcome those challenges. The organically modified clay is known as organoclay and will be described in later sections.
Therefore, there remains a need to develop new methods and compositions to overcome the challenges in obtaining nanocomposites.